The use of multi-chip modules is becoming more and more common in the computer industry. A multi-chip module is an apparatus that allows the interconnection of several integrated circuits without the use of circuit boards with large footprints. Essentially, unpackaged integrated circuits are placed directly on a multi-chip module and pin connections are made using interconnects in the substrate.
Generally, the construction of a multi-chip module begins with a substrate with several connections on one side that can be described as input or output terminals. The input/output terminals may be either solder balls or pins. The terminals are in a grid formation on the substrate and extend through the substrate to metal pads on the other side. An insulating material, such as a polymide, is placed over the pins and various interconnections between pins are made. These interconnections undergo optical inspection for defects and are then typically covered by another insulating layer. Various connections are brought through this insulating layer and then further interconnections are made and, thereafter, optically inspected for defects. This process is generally repeated a number of times, a typical multi-chip module having up to 8 to 100 total insulating layers when completed. Thereafter, integrated circuits are placed on the multi-chip module at the appropriate interconnections. Interconnections through the various layers make up the chip-to-chip interconnects of the multi-chip module.
However, the above process of manufacturing multi-chip modules is not without its problems. Often times defects are overlooked during optical inspection. In addition, defects can occur in a layer after it is optically inspected while additional layers are being added to the multi-chip module.
Consequently, current methods are employed to try to detect any such defects that were missed or created during the manufacturing process when the multi-chip module is completed. One such method involves the use of a capacitive meter. Specifically, a meter probe is placed on one end of an interconnect and the capacitance with respect to ground is determined. However, a capacitive meter employed in this manner is limited by its resolution capability. In particular, such a meter may have difficulty measuring the capacitance of various discontinuities or"opens" in an interconnect.
Another method entails resistive testing. In this method two probes are placed on either end of an interconnect and an attempt is made to pass a current through the interconnect to detect a defect. This method has several problems, however, including difficulty in determining whether a short circuit or a"short" exists between two interconnects. In the case of a short, one may perform a time consuming search for an interconnect which is shorted with another interconnect by process of elimination. Also, the difficulty of synchronizing the placement of two probes onto various interconnects during resistive testing can be difficult and costly to achieve. Often times, testing of multi-chip modules combines both the capacitive and resistive tests, thereby translating into greater complexity, inspection time, and equipment cost.
Yet another method for testing an interconnect is the electron beam test. This test involves charging an interconnect with an electron beam. Thereafter, the interconnect is examined for the size of the charge to see whether leakage has occurred due to shorts or whether opens exist due to discontinuity of the charge through the entire interconnect. However, the cost of the equipment necessary to perform inspection using this method is so high as to be nearly prohibitive.
Finally, another method to determine a defect in an interconnect is by using time domain network analysis (TDNA). In TDNA, a probe is placed on either end of an interconnect and a pulse is propagated through the interconnect. The pulse is reflected from the far end of the interconnect and the reflection is detected. A defect may be determined depending on the time lapse between the pulse transmission and the receipt of the reflection. This method is inadequate as where a short exists, the pulse is ultimately transmitted through two interconnects and a search is necessary to find the interconnect to which the pulse is shorted. As a result, this method may be time consuming, and necessitates high frequency equipment to generate and receive the pulses which translates into significant and even prohibitive cost.
Consequently, there is a need for a testing system and method to test for defects in an multi-chip module that ensures reliable detection of all defects at a minimal cost.